The present invention relates to a method and apparatus for constructing a prosthetic limb socket, and more particularly, to an efficient and consistent method and apparatus for removing a positive cast of a patient's residual limb from within a molded prosthetic limb socket interface after the socket has been molded or laminated over the positive cast.
A prosthesis is often used to replace an amputated portion of a limb and to help restore the amputee's ability to use that limb. A prosthesis for a lower extremity amputation will often include an artificial foot connected to an upright assembly (pylon, tube or shaft) which is in turn connected to a custom fitted socket assembly. If the amputation is above the knee, the upright assembly will commonly also include an artificial knee joint.
The socket assembly of an above the knee prosthesis typically requires two inter-laying sockets: (a) an inner socket consisting of a flexible, thermoplastic or silicone material; and (b) a stronger, less flexible outer socket which is typically formed from a hard thermoplastic or PVC material. The two inter-laying sockets are attached to the upright assembly of the prosthesis. The inner socket is typically designed to interface with and cushion the amputee's residual limb, to protect the amputee's residual limb from the inter-connection components which attach the socket assembly to the upright assembly, and to provide an air tight seal between the residual limb and outer socket. The outer socket is adapted to be coupled to the upright assembly, and provides support to for the patient's residual limb.
The outer sockets are typically created by heating a thermoplastic "preform" cone until it is sufficiently soft and flexible, stretching the heated cone over a positive cast of the amputee's residual limb, and then vacuum forming the cone in place over the positive cast. Alternatively, the outer socket can be fabricated by heating an extruded sheet of thermoplastic material and wrapping the sheet over the positive cast; or by a laminating process using a fiber reinforced, thermal set plastic. It may be desirable to also permanently mold inter-connection components for coupling the outer socket to the upright assembly of the prosthesis into the outer socket during the vacuum forming operation. The inner sockets may also be created by heating a conventional preform cone and by stretching the preform cone over the positive cast of the amputee's residual limb.
Preform cones are typically formed from a clear thermoplastic material, a polypropylene polymer material, a polypropylene homopolymer material, or a flexible thermoplastic material, and are commercially available through Prosthetic Design, Inc., of Clayton, Ohio. Further, a socket manufacturing unit ("SMU"), which is also commercially available through Prosthetic Design, Inc., can be used to heat and mold the preform cones over the positive cast of the residual limb.
Positive casts of the amputee's residual limb are traditionally constructed by taking an impression or negative wrap of the amputee's residual limb, filling the negative impression with a mixture of plaster of Paris, corn starch, vermiculite and water. Once cured, the negative is stripped away and a positive model replica of the patient's residual limb remains. The positive cast is then modified to provide an optimal fit of what will become the prosthetic socket. By adding material or plaster in certain regions of the positive cast a void will be created in the socket which will act to relieve pressure in that area of the patient's residual limb. Likewise, by removing plaster in certain regions of the positive cast, more pressure will be applied to the patients residual limb in that area of the socket interface.
Once the socket material is cured over the positive cast as described above, the positive cast must be removed from the socket interface. Traditionally, the positive cast is chiseled or chipped out using screwdrivers, impact hammers, chisels, etc., until only the socket interface remains. The inter-connection components, the upright assembly and artificial foot are then attached, the prosthesis is aligned unique to each patient, and a prosthetic limb is the result.
Recently, the creation of the positive cast of a patient's residual limb has been performed using computer aided design and manufacturing (CAD/CAM) equipment and software. Such a process usually consists of digitizing a negative impression of the amputee's residual limb, modifying the digitized model of the amputee's residual limb using a CAD software package and then milling the positive cast of the modified digital model on a computer numerically controlled (CNC) machine (otherwise known as milling machines or carvers).
The CNC machine carves the modified positive cast from a "blank" of positive cast material which is commercially available in various materials, sizes and shapes. The majority of present day prosthesis facilities which utilize a CAD/CAM utilize blanks constructed from a formulation of plaster of Paris, corn starch, bleach, water and vermiculite to create their own blanks. Other facilities who utilize a CAD/CAM operation use blanks formulated from a urethane foam formulation.
The positive casts milled from a urethane blank must also be removed from the molded sockets or sleeves. Typically, it is known to remove the urethane positive casts from the sockets by chiseling out the positive cast using screwdrivers, impact hammers, rubber mallets, and chisels (similar to the tools used to remove the plaster casts). It is known to grind the foam casts out using a motorized grinding tool. However, these extraction method tend to be extremely messy and the fine material that is ground or chiseled from the positive casts tends to irritate the skin and respiratory system. Furthermore, the grinding tools have the propensity to scar the inside surface of the socket. It is also known to force the foam casts out utilizing a compressed air method. The compressed air method is preformed by first drilling a small hole in the distal end of the socket and inserting a tube into the hole and forcing compressed air through the tube and into the hole in an effort to force the foam blank out of the socket. A disadvantage with this removal method is if the plastic of the socket is still warm, it will deform to allow the foam blank to be pushed out, therefore the final plastic will not truly represent the positive model. An additional drawback is that the pressure being exerted may be extremely high and dangerous.
Accordingly, a need exists for constructing a prosthetic socket component which facilitates an efficient and consistent method for removing the positive cast of the wearer's residual limb from the socket component after the socket component has been molded over the positive cast.